Option 3 : a divergent lens
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Concept:
- Concave lens: It is a diverging lens that diverging the parallel beam of light.
- It can also gather light from all directions and project it as a parallel beam.
- The focal length of the concave lens is negative.
- It has a virtual focus from the diverging rays of light that seem to converge.
- Convex lens: The lens whose refracting surface is upside is called a convex lens.
- The convex lens is also called a converging lens.
- The focal length of a convex lens is positive.
Explanation:
- The µ of the convex lens is greater than air so it will act as a convergent lens in the air.
- When it is dipped in water its µ is less than water. Now the rays entering and exiting the lens will bend toward or away from the normal in an opposite fashion than when the lens was placed in air.
- So the convergent lens in the air will now behave as a divergent lens in the water.
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